1. Technical Field
The present disclosure is directed to systems and methods for improving fluidization performance. More particularly, the present disclosure is directed to systems and methods for enhancing fluidization of nanoparticles in a fluidized bed, e.g., through reduction in electrostatic charges that would otherwise be present in and negatively impact the performance and/or operation of such a fluidized bed.
2. Background Art
Generation of electrostatic charge is a significant problem when fluidizing dry powders. This problem intensifies if the fluidized powder contains nanoparticles because of the very large specific surface area (surface area per unit mass) in the powder. Indeed, greater levels of electrostatic charge may be generated through triboelectrification, i.e., charge separation associated with the rubbing together of dissimilar (or similar) material surfaces. Such charge separation/electrostatic charge build-up may result from particle-to-particle collisions (similar) and/or collisions between particles and the wall (dissimilar) of the fluidized bed column.
The electrostatic charge generated in a fluidized bed can be particularly problematic with respect to fluidized bed performance for several reasons. First, powder that sticks or adheres to the wall of the fluidized bed can negatively effect fluidized bed performance. FIGS. 1(a), 1(b) and 2 illustrate fluidized bed systems wherein nanopowder has adhered to the wall of the fluidized column due to electrostatic charges. In addition, the formation of clusters of powder that do not fluidize and remain at the bottom of the column, around the distributor and/or around an object placed in the chamber (e.g., a probe) disrupt the flow field in the fluidized bed.
The foregoing phenomena can negatively effect the implementation and performance of a fluidized bed system. For example, if powder sticks/adheres to the wall of the fluidization column or deposits as clusters over the distributor, such powder does not participate in the fluidization process. As a result, transport properties (e.g., heat or mass transfer rates from and to the powder) and reaction rates of a desired chemical reaction (e.g., between the powder and the fluidizing gas), can vary significantly from expected/theoretical levels. These negative effects have been quantified/confirmed by monitoring the moisture adsorbed/desorbed by a fluidized bed of powder. Such testing shows that, when powder sticks to the wall of the column due to electrostatic charge, such powder adsorbs/desorbs less moisture than when the powder is fully fluidized.
Prior efforts have been made to improve the fluidization quality of nanoparticles (in the form of highly porous nanoagglomerates) in a gas fluidized bed. For example, commonly assigned U.S. Patent Publn. No. 2006/0086834 to Pfeffer et al. discloses enhanced fluidization systems that include the introduction of external force and/or pre-treatment to enhance fluidization performance, e.g., sieving, magnetic assistance, vibration, acoustic/sound or rotational/centrifugal forces. Non-provisional patent applications (Ser. Nos. 11/937,736 and 11/937,787; filed Nov. 9, 2007; incorporated herein by reference) to Pfeffer, Quevedo and Flesch disclose the use of microjets to greatly improve the fluidization of so-called ABF type nanoparticles. Further disclosure with respect to external assistance to a fluidization bed is provided by Yu et al., “Enhanced fluidization of nanoparticles in an oscillating magnetic field,” AIChE Journal, Vol. 51, No. 7, pg. 1971 (2005) and Nam et al., “Aerated vibrofluidization of silica nanoparticles”, AIChE Journal, Vol. 50 (8), pp. 1776-1785 (2004). Yu et al. disclose the placement of magnetic particles at the bottom of a fluidized bed of nanoparticles and excitation of such magnetic particles with an oscillating magnetic field while Nam et al. disclose placing the bed on a plate which imparts a vertical sinusoidal vibration to the bed (vibro-fluidization). While these methods help to break-down the large clusters of agglomerates at the bottom of the bed, they also promote the generation of electrostatic charge due to an increase in the overall friction between the particles and between the particles and the inside wall of the fluidized bed.
The addition of water vapor to a fluidization chamber to decrease electrostatic charge and enhance fluidization has been disclosed. For example, reference is made to Yao et al., “Characterization of electrostatic charges in freely bubbling fluidized beds with dielectric particles,” Journal of Electrostatics, Vol. 56, No. 183, pgs. 191-92 (2002). Reference is also made to U.S. Pat. No. 6,946,157 to Folestad et al., entitled “Method and Apparatus for Monitoring the Coating on Particle During Manufacturing of a Pharmaceutical Product,” wherein gas is bubbled through a liquid/solvent before introduction to a coating chamber. However, the solvent vapor introduced into the gas by Folestad et al. is used to enhance drying or deposition of the coating layer, and it is not related to decreasing electrostatic charges. Despite efforts to date, a need remains for systems and methods that provide enhanced fluidization, particularly for fluidization systems that include nanoparticles and/or nanopowders. These and other needs are satisfied by the disclosed systems and methods, as will be apparent from the detailed description which follows.